98 FRANK R. LILLIE. 



the heart, gradually becoming more peripheral in position and 

 slowly fading out (Fig. i). This line represents the junction of 

 the amniogenous and choriogenous somatopleure, and thus cor- 

 responds to the angles of the future amniotic folds. 



The head of the embryo lies in a depression bounded in front 

 by the ectamnion and on the sides by the ammo-cardiac vesicles 

 of the body cavity, along the inner upper margin of which the 

 ectamnion runs for a short distance. The floor of the depression 

 is the proamnion. 



In a stage with 14-15 mesoblastic somites the ectoderm of the 

 proamnion is much more thickened in front of the head, and has 



c . 



FIG. 2. Transverse section through the anterior angle of the ectamnion, a few 

 sections in front of the tip of the'head. 14-15 mesoblastic somites. University of 

 Chicago Embryological Collection, No. 215. b.c. , body-cavity; f., large cavity in 

 the entoderm ; e.ti., ectamnion. 



outer surface in consequence of irregularity in the thick- 

 ening 1 (Fig. 2), which may be traced back to the level of the 

 heart, and on one side to its hinder end ; there is also a very 

 short ectentodermal fusion beneath the tip of the head. In this 

 series the ectamnion marks the boundary between two distinctly 

 differentiated parts of the extraembryonic somatopleure, the more 

 central of which is the amnion. 



In another embryo with fourteen mesoblastic somites, the tip of 

 the head is surrounded by the amnion, and the proamniotic partis 

 represented only by a short median strip extending eight sections 

 back to a point where the limbs of the amnion have not yet 

 closed. The ectamnion is continued only for a short distance 

 along the angles of the amniotic fold, and then passes peripher- 



1 In examining the section one receives a strong impression that the irregularities 

 may be due to amceboid movements ; but it is not possible to confirm this by actual 

 observations. 



